Evaluating the robustness of top coatings comprising plasma-deposited fluorocarbons in electrowetting systems

Thin dielectric stacks comprising a main insulating layer and a hydrophobic top coating are commonly used in low voltage electrowetting systems. However, in most cases, thin dielectrics fail to endure persistent electrowetting testing at high voltages, namely beyond the saturation onset, as electrolysis indicates dielectric failure. Careful sample inspection via optical microscopy revealed possible local delamination of the top coating under high electric fields. Thus, improvement of the adhesion strength of the hydrophobic top coating to the main dielectric is attempted through a plasma-deposited fluorocarbon interlayer. Interestingly enough the proposed dielectric stack exhibited a) resistance to dielectric breakdown, b) higher contact angle modulation range, and c) electrowetting cycle reversibility. Appearance of electrolysis in the saturation regime is inhibited, suggesting the use of this hydrophobic dielectric stack for the design of more efficient electrowetting systems. The possible causes of the improved performance are investigated by nanoscratch characterization.